Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of leaks that could release fuel vapors to the atmosphere. Evaporative leaks may be identified using engine-off natural vacuum (EONV) during conditions when a vehicle engine is not operating. In particular, a fuel system may be isolated at an engine-off event. The pressure in such a fuel system will increase if the tank is heated further (e.g., from hot exhaust or a hot parking surface) as liquid fuel vaporizes. As a fuel tank cools down, a vacuum is generated therein as fuel vapors condense to liquid fuel. Vacuum generation is monitored and leaks identified based on expected vacuum development or expected rates of vacuum development.
In order to preserve battery charge, a typical EONV test is subject to a time limit. A failure to reach a pressure or vacuum threshold before the end of the time limit may result in degradation being indicated, even if the fuel system is intact. For example, the pressure rise portion of the test may execute until the fuel tank pressure curve reaches a zero-slope. If the pressure rise has a relatively low rate of constant increase (e.g., due to cool ambient conditions counteracting the pressure increase), and a significant amount of the time limit elapses prior to a zero-slope moment, the subsequent vacuum test may fail based on the limited amount of time remaining, regardless of the state of the fuel system.
Further, the entry conditions and thresholds for a typical EONV test are based on an inferred total amount of heat rejected into the fuel tank during the prior drive cycle. The inferred amount of heat may be based on engine run-time, integrated mass air flow, miles driven, etc. However, the amount of heat energy transfer needed to reliably assess the integrity of a fuel tank is significantly dependent on the level of fuel in the tank.
One approach for enabling entry into an EONV leak test based on a fuel level is taught by Reddy et al in U.S. Pat. No. 6,321,727 B1. Therein, the EONV test is enabled only when the fuel level is between 15% and 85% of the tank capacity. This fuel level range is specified in order to ensure a suitable vapor volume that is productive of data useful for the diagnostic leak test. Further, ambient temperature is specified to be above 40° F. and fuel tank temperature is specified to be a defined threshold temperature above ambient temperature. If these conditions are not met, the entry into the leak test is aborted. However, while thresholds for fuel tank temperature, ambient temperature, and fuel level are specified for enabling entry into a leak test, the thresholds are static, rather than dynamic, and may thus not always be optimal based on the sum total of current operating conditions. For example, if the level of fuel is near the upper-end of the threshold, yet ambient temperature and fuel tank temperature are near the bottom-end of the threshold, entry into the leak test may be enabled under sub-optimal conditions, potentially resulting in a false test failure. Alternatively, under circumstances wherein fuel is at a level just below threshold yet ambient temperature and fuel tank temperature are significantly above the threshold, entry into leak test may not be enabled even though the likelihood of a robust leak test is high.
The inventors herein have recognized the above issues, and have developed systems and methods to at least partially address them. In one example, a method is provided, comprising, following a vehicle-off event, determining a heat rejection inference for an engine run time duration, and adjusting a heat rejection inference entry threshold based on a current fuel level and a current ambient temperature. For example, a full fuel tank may require a greater heat rejection inference to enable entry into an EONV test than a partially full fuel tank. In this way, EONV tests may be enabled more often than for systems using a relatively high heat rejection threshold, thus maximizing In Use Monitoring Performance (IUMP) rates.
In another example, a method is provided, comprising responsive to and engine heat rejection inference being greater than an adjusted heat rejection inference entry threshold, adjusting one or more pressure thresholds for an engine-off natural vacuum test based on the determined heat rejection inference, isolating the fuel system from atmosphere, and indicating degradation of the fuel system based on the one or more adjusted pressure thresholds. In this way, the leak test parameters may be more indicative of the current operating conditions, decreasing the likelihood of false failures compared to systems using a relatively low heat rejection threshold.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.